1. Field of the Invention
The invention relates to a device and to a method for the determination of imaging errors of an optical imaging system using a radiation-superposition measuring technique which operates with lateral phase offset, for example an interferometric wavefront measuring technique or a Moiré measuring technique, the device having an optical element to be arranged on the object side of the imaging system, having a first periodic structure on the object side with a predetermined periodicity direction, an optical element to be arranged on the image side of the imaging system, having a second periodic structure on the image side with a periodicity direction corresponding to the first periodic structure, the image of the first periodic structure together with the second periodic structure forming an interference or superposition pattern, and a detector to detect the interference or superposition pattern, as well as to a microlithography projection exposure system equipped with such a device. The term object side is here generally intended to mean the region in the beam path preceding the optical imaging system in question, i.e. the specimen, and the term image side is likewise used to denote the entire region in the beam path after the specimen.
2. Description of the Related Art
Such devices and methods for image error measurement are widely known. One application is to determine the imaging errors of high-precision imaging systems, such as those used in microlithography projection exposure systems for the structuring of semiconductor devices. In interferometric wavefront measuring techniques, the detector is usually positioned in a plane, conjugate with the pupil plane of the imaging system, in the light path behind the image plane of the optical imaging system. In the Moiré measuring technique, conversely, the detector is positioned in the image plane of the imaging system, or the image plane is projected onto the detector surface by means of suitable optical elements, for example imaging optics or faceplates. When working with coherent illumination, the detector may also be placed in a Talbot plane behind the image plane.
U.S. Pat. No. 5,991,004 describes a device for defocus determination, which can be used to maintain the positioning of a wafer stage at the focal distance of a microlithography projection objective. To that end, a Moiré superposition pattern is generated by projecting a first Moiré grating onto a second Moiré grating. The second grating is preferably tilted relative to the plane into which the first grating is projected by the objective, so that only one part of the second grating at a time lies at the focus of the objective. A two-dimensional CCD array is used in order to record the Moiré pattern. It detects the movement of rings in the Moiré pattern when the focal plane is shifted, and in particular their movement frequency with respect to the columns of pixels in the CCD array. The movement frequency is then evaluated, for example by means of a fast Fourier transform (FFT), in order to determine the centre of the Moiré pattern and its movement, which is used as a measure of the defocusing.
A wavefront measuring device as disclosed in Patent specification DE 101 09 929 A1, (corresponding to US 2002/0001088A1) which is integrated in a component together with the imaging system to be measured, has an illumination mask on the object side and, on the image side, a diffraction grating as well as a detector having a two-dimensional CCD array, which is read out for the image error determination. In order to measure the image errors, the grating on the image side is successively shifted laterally according to a shear interferometry technique. The phase value calculation and the wavefront reconstruction, as well as the subsequent calculation of the aberration coefficients, are carried out in a suitable evaluation computer. The speed of the measurement is crucially determined by the integration and readout times of the detector, the number of phase steps and the computation time for determining the phase and reconstructing the wavefront.
U.S. Pat. No. 4,659,917 describes a device for focus determination having a plurality of sensor rows. Using a beam splitter, the device divides the exit pupil of an optical imaging system into a multiplicity of exit pupils, the image-generating light rays of which produce a multiplicity of image intensity distributions. The sensor rows respectively comprise a multiplicity of sensors and are arranged at the positions of the image intensity distributions. The sensors deliver output signals corresponding to the intensity distributions, and these are used to detect the relative position of the image intensity distributions so as to determine the focal position, i.e. the image plane position, of the optical imaging system.
U.S. Pat. No. 4,518,854 describes a wavefront sensor which combines features of a wavefront sensor operating by shear interferometry with features of a Hartmann wavefront sensor. Sensor data for measuring the wavefront are obtained both according to the shear interferometry technique and according to the Hartmann pupil splitting principle. The sensor data determined according to the two principles are combined by long-term averaging, which is intended to increase the sensitivity of the wavefront measurement.